1. Field of the Invention
The present invention relates to an organic electrolyte secondary cell which has a high energy density and reliability as a power source for an electronic equipment, for maintaining a memory, for a electric vehicle, for storing electric power and the like.
2. Description of the Related Art
Accompanying with the drastic miniaturization and lightening of electronic equipments, it is highly required to develop a secondary cell as a power source thereof which is miniaturized and lightened as well as has a high energy density, further is capable of charging and discharging repeatedly. In addition, due to environment problems such as air pollution and increase of carbon dioxide, it is desired to utilize an electric automobile in earliest stages. Accordingly, it is desired to develop an excellent secondary cell having features such as high efficiency, high power, high energy density and light in weight. Since secondary cell using an organic electrolyte which satisfies such requirements has an energy density several times as high as that of a conventional cell using an aqueous electrolyte, it is desired to put it to practical use.
As a positive active material of the organic electrolyte secondary cell, various types of material have been examined, such as titanium disulfide, lithium-cobalt composite oxide, spinel type lithium-manganese oxide, vanadium pentoxide and molybdenum trioxide. In these materials, lithium-cobalt composite oxide (LiCoO.sub.2) and spinel type lithium-manganese oxide (LiMn.sub.2 O.sub.4) conduct charging/discharging in extremely high potential more than 4 V (Li/Li.sup.+). Consequently, they are used as a positive electrode so as to utilize a cell having a high discharge voltage.
As a negative active material of the organic electrolyte secondary cell, lithium, Li-Al alloy and carbon material capable of occluding and discharging lithium ion, and the like have been examined. In these materials, carbon material has an advantage that a cell having a long cycle life can be obtained.
However, in this kind of cell, since lithium having lower potential is used as the negative active material and metal oxide having higher potential is used as the positive material, electrolyte is easy to be decomposed. Accordingly, it is necessary to consider about this point to select the electrolyte, and various kinds of electrolytes have been proposed to use. Almost all of the electrolytes are the mixture of a high dielectric constant solvent such as propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, sulforane, and a low viscosity solvent such as 1,2-dimethoxyethane, dimethylcarbonate, ethylmethylcarbonate, diethylecarbonate.
On the other hand, as a solute, lithium perchlorate, lithium trifluoromethanesulfonate, lithium hexafluorophosphate and the like are generally used. Particularly, lithium hexafluorophosphate is popularly used in recent, because of high safety and high ion conductive rate of electrolyte in which it is dissolved.
However, when carbon material is used as the negative electrode, a reduction decomposition reaction of the electrolyte occurs on the surface of the negative electrode with generating gas in the first charging. Accordingly, a cell case may be swelled, or a cell capacity may be reduced.
The charge is forwarded to stop generating gas, so that a charge reaction to carbon begin to forward. That is, a electrolytic polymerization reaction occurs on the surface of the carbon material in the initial stage of charge, and a polymer coat is formed on the surface of the carbon material. When the coat is formed to some degree, the electrolytic polymerization reaction is supressed because of lack of the electron conductivity of the coat, thereby forwarding only charging reaction of lithium ion. However, since lithium ion is consumed for the polymerization reaction in the initial stage, and is not effectively used for charging reaction, the capacity of the cell is reduced.